1. Field of the Invention
The present invention relates to a novel non-flammable mixed solvent composition, and a cleaning method and a cleaning apparatus using such a solvent composition, and further particularly to a novel nonflammable mixed solvent composition which is replaceable for organochlorine solvents including 1,1,2-trichloro-1,2,2-trifluoroethane (hereinafter abbreviated as "flon 113") and 1,1,1-trichloroethane (hereinafter abbreviated as "triethane") and has excellent properties, in particular, as a cleaning solvent and a vapor drying solvent (vapor cleaning and drying solvent), and a cleaning method using such a solvent and a cleaning apparatus for performing the cleaning method.
2. Related Background Art
Chlorofluorocarbons (hereinafter abbreviated as "CFC") have heretofore been used in many industrial fields because most of them are low in toxicity, incombustible and chemically stable, and various kinds of flons different in boiling point are available.
Among these, flon 113 has been used as a degreasing solvent or vapor drying solvent for many plastic materials, a part of rubber materials and various composite materials, making good use of specific chemical properties inherent in the solvent.
The reason why flon 113 has heretofore been often used as a vapor drying solvent is that it has the following properties:
a) to have moderate dissolvability for various kinds of dirt and oils; PA1 b) to have excellent chemical and thermal stability, and so there is little fear of decomposition; PA1 c) to be low in toxicity and have neither flash point nor ignition point, and hence be high in operational safety; PA1 d) to be low in polarity and hence cause no damage to various kinds of plastic materials; and PA1 e) to have a sufficiently low surface tension and hence wet the surface of a substance to be cleaned to thinly and evenly spread thereon. PA1 a) to have strong detergency to metalworking oils; PA1 b) to be low in toxicity and have neither flash point nor ignition point, and hence be high in operational safety; and PA1 c) to cause no damage to metals to be cleaned because it has no water-solubility and hence contains no water therein (namely, low in possibility that it may oxidize and corrode the metals).
Therefore, in order to replace flon 113, a replacement is also required to have the same properties as described above.
At present, hydrochlorofluorocarbons such as 1,1-dichloro-2,2,2-trifluoroethane (hereinafter abbreviated as "HCFC123"), 1-fluoro-1,1-dichloroethane (hereinafter abbreviated as "HCFC141b"), 1,1-dichloro-2,2,3,3,3-pentafluoropropane (hereinafter abbreviated as "HCFC225ca") and 1,3-dichloro-1,1,2,2,3-pentafluoropropane (hereinafter abbreviated as "HCFC225cb") have been developed as second-generation flons and are under investigation as to whether they can be used as replacements or not.
However, these second-generation flons are high in dissolvability for various kinds of oils and fats, but most of them attack general-purpose plastics such as acrylic resins and ABS resins and besides, they are high in ozone depletion potential because they contain chlorine in their molecules. Therefore, they are not of serious consideration as replacements.
Besides, triethane has been used as a replacement for trichloroethylene and tetrachloroethylene, which have hitherto been used as degreasing and cleaning solvents for metal surfaces after metal working, are noxious to the human body and form the cause of pollution of underground water.
The reason why triethane has been used is that it has the following properties:
At present, methylene chloride (dichloromethane) and various petroleum cleaning solvents have been studied as replacements for triethane. However, no replacement well balanced among all the aspects of safety (toxicity, carcinogenesis, flammability, etc.), detergency and drying characteristics has been yet developed.
A method in which only a completely fluorinated organic compound is used as a vapor drying solvent is also studied in a semiconductor-fabricating process and the like. However, completely fluorinated organic compounds involve a problem that since they have no degreasing ability, a stain tends to occur on the surface of an article cleaned unless the article is completely degreased prior to this cleaning step.
The completely fluorinated organic compounds exhibit specific dissolution behavior that they are miscible only with some limited solvents. Therefore, they involve a problem that a stain after drying tends to occur because a solvent and water used for degreasing in a preceding step cannot be purged with ease.
In recent years, ozone holes by which global environment is made worse have been discovered, and the main cause that the ozone hole is created has been proved to be attributable to organochlorine compounds.
For example, flon 113, which is chemically stable, lasts long in the troposphere, diffuses to the stratosphere and is photochemically decomposed by sunrays there, thereby generating chlorine radicals which then combine to ozone so as to destroy the ozonosphere. Therefore, after this, the organochlorine compounds including flons have a tendency for their use to be internationally limited or prohibited.
Among these, flon 113 as described above is high in ozone depletion potential. Thus, flon 113 is desired to be early replaced with proper one, and has a schedule for unsparingly reducing its use. Triethane and other organochlorine solvents are also scheduled to be unsparingly reduced for the same reason as described above.